Gas turbines are well-known mechanisms for generating electrical power. The turbine is driven by a heated compressed working gas to drive a shaft which powers an electrical generator. The working gas is heated by heat produced from combustion of air and a fuel and is compressed by a compressor coupled to the turbine. Traditional gas turbines are often limited to expensive refined fuels, such as distillate oil and natural gas.
A type of known gas turbine is an indirect-fired gas turbine wherein the working gas is kept separate from the combustion products by a heat exchanger that transfers heat generated by the combustion process to the working gas. The heat exchanger is located inside a separate furnace wherein the combustion of the fuel and air occurs. This indirect cycle duplicates the Brayton cycle that occurs in conventional gas turbine operations. Such indirect-fired gas turbines are particularly attractive as they do not expose the turbine components to harmful combustion products. Further, it is possible to utilize less refined and consequently less expensive fuels such as coal, heavy oil, wood products and other biomass, solution gas and combustible waste material in the combustion process (hereinafter “unrefined fuels”).
The heat exchanger has conduits in the furnace which will have surface temperatures far above what is typical for conventional steam boiler systems. This high temperature is achieved by direct radiant heating from a luminous flame in a primary combustion chamber of the furnace. The aforementioned heavier fuels will typically burn with highly luminous flames that more readily radiate thermal radiation than lighter fuels like natural gas, propane or refined liquids. Thus, heavier fuels are advantageous for higher rates of heat transfer. One disadvantage of using unrefined heavier fuels is that the luminous flames from these fuels tend to contain one or a mixture of highly reactive radicals, corrosive sulphur compounds and other radicals, as well as fouling substances like metals, soot, slag, ash or un-burnt fuel. Such particles when contacting the heat exchanger conduits can be harmful to and reduce the thermal effectiveness and lifespan of these conduits.